1. Field of the Invention
This invention relates to a heavy duty pneumatic tire for use in tuck and bus, and more particularly to a heavy duty pneumatic radial tire having a tread pattern suitable for reducing tire damage due to stone capturing.
2. Related Art Statement
When a heavy duty pneumatic tire of this type is run at such a state that stones scattered on road are held in grooves of the tread portion, the groove bottom portion is damaged due to the loading repeatedly applied to the groove portion capturing a stone, and further such a damage may grow inside the tire.
In order to prevent tire damage due to such stone capturing, there has hitherto been known a heavy duty pneumatic tire as shown in FIGS. 1a and 1b, wherein protrusions 14 for the prevention of stone capturing are disposed in a bottom of a circumferential groove 12 substantially extending in a circumferential direction of a tire 10 along the groove extending direction so as to suppress stone capturing in the circumferential groove 12 and discharge the captured stone from this groove through the elastic force of the protrusion 14 during running.
Moreover, a transverse groove 16 crossing with the circumferential groove 12 may be arranged in the tread of such a heavy duty pneumatic tire so as to improve traction and braking performances. In this case, the stone capturing in the transverse groove is naturally anticipated. However, a stone captured by the transverse groove is easily discharged by the stepping-in and kicking out motion of tread rubber during the running. If the protrusion for the prevention of stone capturing is disposed in the bottom of the transverse groove, the radius of the groove bottom becomes small and consequently cracking is apt to be caused, so that the arrangement of the protrusion is not usually made in the transverse groove.
Even in such a tire provided with the protrusions for the prevention of stone capturing, however, intersect portions between the circumferential groove 12 and the transverse groove 16, e.g. regions A and B are composed of sides and corner portions of three or four blocks constituting island portions of the tread, respectively, so that when stone is captured by the intersect portion at once, there is also caused the aforementioned problem resulting from the stone capturing because the captured stone is elastically engaged with any of the rubber blocks and the protrusions 14 for the prevention of stone capturing disposed along the circumferential groove 12 can not sufficiently develop its function to maintain the stones at the captured state. Moreover, such a problem is known to be caused irrespective of the tire structure.
In the heavy duty pneumatic tire having the block pattern as in the above conventional tire, shoulder wear is apt to be caused in the blocks at the shoulder portion of the tread, while heel and toe wear is apt to be caused in the blocks of the central portion of the tire. For this end, a blind sipe not extending to the transverse groove and the circumferential groove is formed in each block as shown in FIG. 1a. Thus, the rigidity of each block is lowered by the blind sipe to reduce any shearing force produced at the ground contact area of the tread in the circumferential direction during the stepping-in and kicking-out motion, whereby the heel and toe wear is controlled.
In this heavy duty pneumatic tire, however, there is still a problem that the shoulder wear is apt to be caused at the blocks of the shoulder portion most influenced by lateral force.